1. Field of the Invention
The present invention relates to an ionic conductive polymer electrolyte and a cell comprising the same. More particularly, the present invention relates to a lithium ionic conductive polymer electrolyte which can be used as an electrolyte for a lithium cell or an electrochromic display or in a sensor for lithium ion concentration or a lithium ion separating film.
2. Description of the Related Art
Many attempts have been made on an application of a polymer electrolyte which is flexible and formed in a film as a lithium ionic conductive solid electrolyte of a lithium cell and the like.
The polymer electrolyte consists of a complex comprising a lithium salt and an organic polymer in which the lithium salt is dissolved. When the polymer electrolyte is used in the lithium cell which is required to be thin and small by utilizing its characteristic that it can be easily formed in a film, it can improve the workability in the production of the cells and shielding effects and serve for reducing a production cost of the cell. Because of its flexibility, the polymer electrolyte may be useful as an electrolyte for an electrochromic display or as a sensor for lithium ion concentration.
As the organic polymer which constitutes the polymer electrolyte, many polymers are proposed such as polyethylene oxide (H. B. Armand, Fast Ion Transport in Solid, 131 (1979)), polyethyleneimine (T. Takahashi et al, Solid State Ionics, 18 & 19, 321 (1986)), polyethylene succinate (M. Watanabe et al, Macromolecules, 17, 2902 (1984)), cross-linked triol polyethylene oxide (Polymer Journal, 18(11), 809 (1986)) and the like.
Since the conventional polymer electrolytes have low lithium ion conductivity of 1.times.10.sup.-8 to 1.times.10.sup.-5 S/cm at 25.degree. C., the lithium cell or other apparatus comprising it does not exhibit its performances satisfactorily when it is used at room temperature.
The ionic conductivity of the polymer electrolyte is induced by segmental motion of the polymer as suggested by D. F. Shriver et al (C. & En., 59 (1985)). The segmental motion of the polymer can relate to the free-volume, and the following ionic conduction equation is proposed (T. Miyamoto et al, J. Appl. Phys., 44(12) 5372 (1973) and M. Watanabe et al, J. Appl. Phys., 57 123 (1985)): ##EQU1## wherein q: an electric charge,
n: the number of ionic carriers, PA1 .mu.: an ionic mobility, PA1 n.sub.0 : a constant, PA1 W: a dissociation energy of a salt, PA1 .epsilon.: a relative dielectric constant of a polymer, PA1 k: the Boltzmann's constant, PA1 q.sub.0 : a constant, PA1 D: a diffusion constant, PA1 r: a numerical factor, PA1 v.sub.i : a critical hole required fro ion motion, PA1 v.sub.g : a specific volume of at T.sub.g, PA1 f.sub.g : a free-volume fraction at T.sub.g, PA1 .alpha.: a thermal expansion coefficient of the free volume, PA1 T.sub.g : a glass transition temperature.
To increase the conductivity, the ionic mobility (.mu.), which more greatly contributes to the ionic conductivity than the ionic concentration (n), should be increased. To this end, it is necessary to decrease the glass transition temperature (T.sub.g) or to increase the specific volume (v.sub.g) at T.sub.g, namely, to decrease crystallinity of the polymer. This is supported by the facts that P. M. Blonsky et al obtained a liquid polymer electrolyte having an ionic conductivity of 1.times.10.sup.-4 S/cm by using polyphosphazene having T.sub.g of -83.degree. C. which is lower than T.sub.g (-60.degree. C.) of polyethylene oxide (Solid State Ionics, 18819, 258 (1986)) and that M. Watanabe et al obtained a polymer electrolyte having an ionic conductivity of 1.times.10.sup.-5 S/cm by using cross-linked triol polyethylene oxide having the crystallinity of 30% comparison to he crystallinity of 70% of polyethylene oxide (Polymer Journal, 18(11), 809 (1986)).